1. Field of the Invention
The present invention relates to a scanning image displaying apparatus for displaying an image by scanning light by optical scanning means, and particularly to a displaying apparatus suitably usable in a head mounted image display apparatus for displaying a predetermined image based on an image signal, and an electronic view finder system of a digital camera or the like.
2. Related Background Art
At the present days, as an image displaying apparatus such as a head mounted image display apparatus (a so-called HMD) and a digital camera, there exists an apparatus that uses a two-dimensional displaying element, i.e., a so-called flat panel, such as a transmission liquid crystal element, a reflection liquid crystal element, or an organic EL (electroluminescent) element. A variety of image displaying apparatuses have been proposed, in each of which the two-dimensional displaying element and an eyepiece optical system are combined such that a display image can be observed as a virtual image. In recent years, a higher-precision image is required in such image displaying apparatuses. Accordingly, a very large number of pixels must be provided on a flat panel display, and defects of pixels are likely to increase as the number of pixels increases. Further, the size of a pixel decreases relative to the size of a flat panel. Thus, problems of difficulty in manufacturing and the like occur.
On the other hand, a displaying apparatus using scanning means in place of a two-dimensional displaying element has been disclosed and proposed in U.S. Pat. No. 5,467,104. This U.S. Patent discloses techniques for scanning light of three colors, i.e., R(red), G(green) and B(blue), in both horizontal and vertical directions to form an image directly on a retina through an optical system.
As semiconductor process techniques advance as a method of fabricating scanning means for scanning a light beam, it becomes possible to obtain techniques for producing scanning means, which is high in speed while small in size and light in weight, by using techniques of a micro electro mechanical system (MEMS). Japanese Patent Application Laid-Open-No. H07-175005 (its title is “Planar type mirror galvanometer and method of manufacture”, its applicant is The Nippon Signal Co., Ltd, and its U.S. counterparts is U.S. Pat. No. 5,606,447), and Japanese Patent Application Laid-Open No. H08-334723 (its title is “Optical deflecting device”, and its applicant is Olympus Corporation) disclose and propose scanning means fabricated by using semiconductor process techniques. Likewise, MEMS of two-dimensional scanning means usable in an HMD is proposed by SPIE, Conference #4407 19(June 2001), “Wafer scale packaging for a MEMS video scanner”, and a small-sized scanning image displaying apparatus can be achieved by using this scanning means.
Also in the scanning image displaying apparatus disclosed in the above-mentioned U.S. Pat. No. 5,467,104, light is required to scan at a very high speed, so that a very small device is used in a scanning portion, such as a mirror, for scanning light. In the event that such a small-sized scanning device is used, a scanned light beam becomes very thin, and hence the diameter of the light beam at the position of a retina of an observer becomes exceedingly small. Therefore, its exit pupil also becomes small, and it causes a problem that observation of an image is likely to be impossible when the positional relationship between an optical path and an eyeball varies.
As a method of giving an image to an observer in a scanning image displaying apparatus for solving the above-discussed problem, there exist techniques disclosed in U.S. Pat. Nos. 5,701,132 and 5,757,544. In an apparatus of U.S. Pat. No. 5,701,132, expanding means, such as a lens array and a diffuser, is disposed on an intermediate image plane, on which a scanned beam forms an image, light is transmitted through the expanding means, and a span angle of the light beam transmitted through the expanding means is enlarged.
Furthermore, adjustment of a diopter is required since there are a nearsighted person and a farsighted person among observers. In general, for purposes of adjusting the diopter using those image displaying devices, the diopter adjustment is carried out by varying the positional relationship between an image displaying device and an eyepiece optical system, or providing an eyepiece optical system for adjusting a diopter and moving this optical system.
In a method of adjusting a diopter, a movable optical system for correcting a diopter is provided, but in this case it is likely that the number of members increases and the size of an apparatus becomes large. Accordingly, in the event that a two-dimensional displaying device is used, a method that is adopted in which the device itself is moved to adjust a diopter comes in handy.
In a scanning image displaying apparatus, when a diopter is to be adjusted by moving a scanned surface (a surface to be scanned) which is equivalent to a two-dimensional displaying device, a problem that an image on the scanned surface is likely to blur occurs. A reason for a conjugate relationship between the light source and the scanned surface is broken if the scanned surface is moved.